Antisera to protein kinase C (PKC) have been used to examine the presence and distribution of the enzyme in developing cerebellar cortex of postnatal rat and in cultures of rat sympathetic ganglia. In the cerebellar cortex of 2-,4-, and 6-day old rats, immunostaining was observed in areas of early-forming presynaptic terminals and growth cones. No staining was evident in the cortical proliferative zone. Beginning at 10 days postnatal, nuclear staining, not apparent at earlier stages, was prominent in Purkinje cells. In neuronal cultures of dissociated rat sympathetic ganglia, PKC was immunolocalized in cell bodies and bundles of neuronal processes. Immunoreactivity was particularly striking in growth cones of extending neurites and in axonal varicosities. These results suggest a role for PKC in neuronal growth following cell proliferation and in synaptic function. The appearance of nuclear staining in later developmental stages suggests that the enzyme may be involved in the promotion and maintenance of the differentiated state of neurons.